BEIRUT — Air attacks by the Syrian government killed at least 25 people Wednesday in the rebel-controlled town of Azaz in one of the bloodiest days in the country’s conflict, activists said.

The town, north of the city of Aleppo and a few miles from the Turkish border, was pounded for hours by warplanes that left homes flattened and about 200 people injured, government opponents said. Azaz has been the target of many previous attacks by forces loyal to President Bashar Assad.

The deaths were among almost 200 reported across Syria on Wednesday, and they included 23 people slain execution-style in fields in the suburbs of the capital, Damascus, activists said.

Violence also spilled into neighboring Lebanon, where a powerful Shiite clan that backs Assad said it abducted at least 20 Syrians in retaliation for rebels holding one of their relatives captive in Syria. The rebels accuse the Lebanese man of belonging to Hezbollah, a Shiite Lebanese group allied with Syria and Iran.

Speaking on Lebanese TV, relatives of Hassane Salim al-Mikdad, who was shown earlier in an online video being held by a group of armed rebel fighters, said they have responded to the kidnapping in a similar manner.

“For Hassane, we are willing to kidnap 2 million Syrians,” one relative said.

The Lebanese information ministry quoted al-Mikdad’s relatives as vowing to continue kidnapping until he is released.

Lebanese President Michel Suleiman warned against threatening the safety of those inside Lebanon, regardless of national origin. “Spreading of chaos does not bring back any kidnapped nor restore any right; instead, it shatters all rights and endangers the lives of citizens,” he said.

Saudi Arabia and the United Arab Emirates, which have been supporting the Syrian rebels, warned their citizens to leave Lebanon.

Meanwhile, three people were injured in a bombing Wednesday in Damascus that struck near a military compound and the hotel where United Nations monitors have been staying.

The Free Syrian Army took responsibility for the blast, which reportedly targeted a meeting of military officers at the compound in the heavily guarded Arkan neighborhood of Damascus and suggested that top officials remain vulnerable to such rebel attacks.

No U.N. monitors were injured.

Also Wednesday, a U.N. Human Rights Council panel reported that Syrian government forces and allied militias have waged a brutal campaign of murder, torture, rape and indiscriminate shelling of Syrian civilians. The actions amount to war crimes and crimes against humanity, the panel said.

Rebels also committed war crimes, including murder, torture and unlawful killings, the panel concluded, but not on the same scale or with the same frequency as government forces, the panel determined.

Activists say more than 17,000 people have died in the violence; the Syrian government says about 8,000 government forces and civilians had died as of early July.